This invention relates to high quality specular mirrors, and more particularly to a mirror comprising a polished aluminum substrate with a gold surface coating.
Aluminum mirrors are used in a wide variety of optics systems, particularly those used in guided missiles. Such mirrors must be able to be formed to fine tolerances, and yet withstand the harsh conditions within a missile nosecone. Minimizing weight is also an important factor in such systems.
Because of the oxidizing atmosphere to which the mirrors are exposed, it is not possible to use an uncoated aluminum surface because aluminum readily oxidizes and loses its reflectance. It has been found that a gold surface can give the high reflectance required in such systems, while the gold is effectively inert to oxidation or corrosion in its working atmospheres.
Therefore an ideal mirror might be formed from aluminum with a protective and highly reflective gold coating applied directly to the aluminum surface. Unfortunately, it has been found that gold will not adhere well to aluminum, and therefore that such a gold coating easily flakes off the aluminum surface.
One solution to this problem has been to form a mirror with an aluminum substrate and a gold surface, with suitable intermediate layers to hold the gold coating in place. In one such mirror, the aluminum substrate is formed to the approximate required shape and is then plated with nickel. The nickel is then precisely ground to the desired shape, and is then coated with chromium. The chromium is then coated with the reflective gold. The chromium is used because gold seems to adhere very well to it, and it in turn adheres well to the nickel. However, there are several problems with using nickel as an intermediate layer in such mirrors. First, nickel and aluminum have a large difference in electronegativities. As a result, there tends to be a high amount of corrosion at the nickel-aluminum interface whenever oxygen diffuses into the surface. Therefore, a relatively thick coating of nickel must be applied to the aluminum to protect against such corrosion. This layer of nickel must then be machined and polished to specular reflectance. The thick nickel layer also adds a considerable amount of weight to the mirror.
Previously it had been very difficult to machine and polish soft aluminum to a surface which could provide high quality specular reflectivity. Therefore the nickel served the additional purpose of providing a hard surface which could be polished to specular reflectance. Two machining operations were required in making such mirrors. First the aluminum is machined to the approximate surface proportions. Then the nickel must be machined to the exact required shape. Recently, however, laser controlled diamond turning equipment has been developed which is now capable of forming a high quality specular reflective surface directly on aluminum. Therefore, there is no longer any need to have a nickel surface to provide the desired reflective surface, and it would therefore be desired to eliminate the thick electroplated nickel layer.
As previously mentioned, the gold cannot be deposited directly on the aluminum, but needs to be deposited on a chromium substrate to which it adheres well. However, just as electrolytic corrosion occurs at the nickel-aluminum interface, so such corrosion will also occur at a chromium-aluminum interface because of the high difference in electronegativity.